1. Field of the Invention
The invention relates to a method for protecting a portable card, provided with at least a crypto algorithm for enciphering data and/or authenticating the card, against deriving the secret key through statistical analysis of its information leaking away to the outside world in the event of cryptographic operations, such as power consumption data, electromagnetic radiation and the like. The card is provided with at least a shift register having a linear and a non-linear feedback function for creating cryptographic algorithms. The method comprises loading data to be processed and a secret key in the shift register of the card.
2. Description of the Prior Art
Using a secret key to process input information and/or to produce output information is generally known in the event of cryptographic devices. Using feedback shift registers is also generally known for creating cryptographic algorithms.
In this connection, data to be consecutively processed and a secret key are loaded into one or more shift registers. Here, the sequence of loading data and the key is random.
Subsequently, the output of the shift register and possibly the shift-register contents are applied, using linear and/or non-linear-feedback, to determine the output of the entire algorithm. The input of the shift register then, apart from the data and the key, also consists of a linear and a non-linear combination of the shift-register contents.
Such shift registers are generally applied in the event of portable cards, such as chip cards, calling cards, smart-card products and the like.
Since the secret key is not known to unauthorized third parties, it is basically impossible to derive either the input or the key from the output of the algorithm.
Now it has become apparent, however, that for chip cards and the like it is possible, in the event of computations, to derive the secret key used from a statistical analysis of the power consumption of the card. Such methods are known as “Differential Power Analysis” (=DPA) and are described in the Internet publication DPA Technical Information: “Introduction to Differential Power Analysis and Related Attacks” by P. Kocher et al., Cryptography Research, San Francisco, 1998.
Such methods are based on the fact that, in practice, with cryptographic operations, information is leaking away to the outside world in the form of power-consumption data, electromagnetic radiation and the like.
Thus, logical microprocessor units show regular transistor-switching patterns which externally (i.e., outside the microprocessor) noticeably produce electrical behaviour.
In this manner, it is possible to identify macro characteristics, such as microprocessor activity, by recording the power consumption and deriving information on the secret key used by way of statistical analysis of the data thus obtained.